To control loudspeaker variability, some manufacturers test every loudspeaker in their production. This test ensures a nominal speaker and enclosure variability. A reasonably large tolerance window or high cost loud speaker is required to obtain a reasonable pass rate.
To boost speaker performance, electrical equalisation is often applied. However, this can only be an average curve since individual adjustment would be prohibitively expensive.
At high power levels or any other times where a speaker may exhibit non-linearities, few if any inexpensive solutions exist. One solution in high power, high performance systems is to mount an accelerometer on the speaker driver, and use a feedback system to linearise the driver. Myers discloses such a system in U.S. Pat. No. 3,798,374, issued on Mar. 19, 1974.
In attempting to avoid such costs, other convoluted circuitry has been used to monitor the drive signal of the loudspeaker. This added circuitry adds to the expense and a great deal must be known about the driver before it can yield any useful information. Bai and Wu describe such a circuit in “Robust Control of a Sensorless Bass-Enhanced Moving-Coil Loudspeaker System”, J. Acoust. Soc. Am. 105(6) pp. 3283–3289.
It is evident that, if one wants to avoid non-linearities (adaptive acoustic echo cancellers do not tolerate them), a cost penalty is imposed. This cost penalty comes about either as increased component cost, increased test time, reduced production yield, extra transducers, or extra circuitry. Even with these extra costs, non-linearities are unavoidable since clipping, buzzes and rattles are still possible.